1. Field of Invention
The present invention relates to steel products having superior weathering, i.e., atmospheric corrosion resistance, a method of forming weathering protective rust on surfaces of steel products in a short period of time, and a method of producing the steel products having superior weathering based on the method of forming weathering protective rust.
2. Description of Related Art
Weathered steels containing one or more alloy elements, such as P, Cu, Cr and Ni, which are added in the steel to improve corrosion resistance in the atmosphere, are widely employed in steel structures such as ships and tanks, and steel buildings, such as bridges. In weathered steel, the so-called protective rust, i.e., rust hardly permeable to oxygen and water that are responsible for corrosion, is formed on steel surfaces after several years outdoors so that subsequent rusting is inhibited. Therefore, weathered steel is free from the need of coating with a corrosion-resistant paint, and thus provides an inexpensive and highly corrosion-resistant materials that can be used as is without painting. Also, the protective rust formed on the weathered steel is dark brown in color in comparison with reddish rust generated on plain steel, and is hence more harmonious with its surroundings. For those reasons, weathered steel has been more commonly used in recent years.
However, weathered steel requires a long period of time to manufacture, i.e., several years, to form the protective rust and generate flowed rust, called rust liquid, during that period. This raises problems from the point of harmonizing the weathered steel with its surroundings as well as causing environmental pollution. Another problem occurs in coastal areas because the protective rust is difficult to grow on weathered steel due to the action of salt particles flying from the sea, and, therefore, inhibition of rusting does not result.
To solve the problems described above, several proposals have been made. For example, Japanese Unexamined Patent Publication No. 49-11739 proposes steel products produced by forming rust on steel materials under an action of one or both of water and a corrosive liquid, and then forming a film, which is semipermeable to water on the rusted steel materials.
Japanese Unexamined Patent Publication No. 1-142088 discloses a surface treating method for weathered steel by which a surface of a steel plate is treated by an acid solution containing iron ions, and a phosphate coating is then formed on the treated surface.
Japanese Examined Patent Publication No. 7-37672 discloses a rusted weathered steel plate which is treated with a rusting liquid having a special composition.
Japanese Unexamined Patent Publication No. 6-136557 discloses a surface treating method for steel materials with which an aqueous solution of chromium sulfate or copper sulfate is coated on steel materials and after drying of moisture, an organic resin coating is formed on the steel materials. Further, Japanese Unexamined Patent Publication No. 8-13158 describes a surface treating method for steel materials with which an aqueous solution containing aluminum ions is coated on steel materials and after drying of moisture, an organic resin coating is formed on the steel materials.
Japanese Patent No. 257247 discloses steel products having high weathering in coastal areas, which contain very small amounts of Si and S, are combined with Ni, Al and Nb and contain a composite oxide of Al and Ca.
Additionally, Japanese Unexamined Patent Publication No. 6-264256 discloses steel products having high weathering wherein surfaces of the steel product are covered by a rust layer having a crystal grain size not larger than 200 nm in terms of means crystal grain diameter.
The related techniques described above, however, have several problems as described below. With the techniques disclosed in Japanese Unexamined Patent Publication No. 49-11739 and Japanese Examined Patent Publication No. 7-37672, when the steel products are exposed to the atmosphere, corrosion is inhibited in an initial stage, but rusting progresses after the initial stage. It is, hence, impossible to perfectly prevent formation of flowed rust and the effluence of iron ions. In addition, both of the steel products have problems associated with handling the materials because treatment liquids mixed with chemicals are used in the production process.
With the techniques disclosed in Japanese Unexamined Patent Publication No. 1-142088, weathering rust is formed in a shorter period of time than the predecessor techniques, but the coating color is black in an initial stage. Therefore, rusting progresses at different speeds due to a difference in the exposure environment between places that are exposed to direct sunlight and those in the shade. This makes the surface color non-uniform and raises a problem with the appearance of the material.
With the techniques disclosed in Japanese Unexamined Patent Publication No. 6-136557 and No. 8-13158, weathering the rust is developed in a short period of time, but problems are experienced due to complexity of the treatment process and the high cost of the surface treatment solution. Furthermore, the steel products produced in accordance with the technique disclosed in Japanese Patent No. 257447 have the problem that the surface color becomes non-uniform depending on environmental conditions and thus, the weathering of the steel products is insufficient.